Spinets, in general, are a class of minerals having the general formula A2+B23+O42−. Aluminum spinels include natural spinels, MgAl2O4, gahnite, ZnAl2O4, and hercynite, FeAl2O4. Although spinels are found in nature, because of the increasing demand synthetic spinels are now being made by various methods.
Presently, spinels are used as windows and domes in high-speed missiles and pods. Although silicate glasses are suitable for most common window uses, they are too weak to be used in the latter applications. A magnesium aluminate spinel is a polycrystalline ceramic material that has excellent optical and mechanical properties.
In the spinel technology area, it is well known that the production of high purity, high transparency spinels is generally accomplished by coprecipitation of aluminum and magnesium salts. This can be accomplished by various means including by milling aluminum and magnesium compounds together to obtain a homogeneous mixture of materials, or simply wet mixing high purity aluminum and magnesium compounds. In other words a spinel containing suspension including both the magnesium compound and the aluminum compound is prepared. This compound suspension(s) may then be hydrothermally aged. A drying step followed by a calcination step then occurs. The drying step typically includes spray drying. Spray drying is a well known technique and sophisticated spray driers with various nozzle types are known.
To be noted however, is that when making spinels it is the magnesium, aluminum containing suspension that is fed into a spray dryer. This method is disclosed, for example in the following documents; V. Montouillout et al., J. Am. Ceram. Soc. 82(12) 3299-304 (1999), G. Lallemand et al., J. Eur. Ceram. Soc. 18(14) 2095-2100 (1998), and W. K. Zhang et al., J. of Alloys and Compounds 465 250-254 (2008).
Prior art discloses the use of spray drying equipment that includes nozzles that can combine two liquid streams. The prior art discloses that the two liquid streams are allowed to be of different compositions, for example, solvents of different boiling points or reactant solutions. The prior art discloses that plugging of the nozzles is a problem and offers various solutions of improved nozzles to address this problem. However in the spinel technology area as explained above, it is the mixed aluminum and magnesium containing suspension that is fed into the spray dryer. This is because many synthetic routes for spinel include precipitation from magnesium and aluminium salts. The high concentration and aggressive nature of the salts limits the materials of construction, and the in situ precipitation of the materials has deleterious effects on the dryer due to the propensity of the suspension to gel under many conditions. Even in the cases of mixed oxides, which are generally less reactive, the materials are generally premixed when spray dried for viscosity control reasons. Without careful control of the suspensions, there is a tendency for the materials to thicken rapidly.
The inventors of the present invention have also found that these prior art spinel production processes are problematic during scale up, i.e. to an industrial or commercial scale. Without wishing to be bound to any theory, it is believed that this is due to a limited weight loading at which the aluminum and magnesium containing suspension can be processed by a spray dryer. Typically the suspension can only be made at up to 7 to 9 wt. % loadings before the nozzles begin to plug or the suspension becomes too viscous to pump. Such a low loading limit creates a bottleneck at the spray drying phase and thus increases the time required to produce commercial quantities of the spinels.